drm/i915/fbc: unconditionally update FBC during atomic commits

We unconditionally disable/update FBC even during the page flip
IOCTLs, and an unconditional disable/update at every atomic commit
touching the primary plane shouldn't impact PC state residency
noticeably. Besides, the code that checks for rotation is a good hint
that we may be forgetting something else, so let's leave all the
decisions to intel_fbc.c, making the code much safer.

Once we have the code to properly make FBC enable/update decisions
based on atomic states, with proper locking, then we'll be able to
evaluate whether it will be worth trying to optimize the cases where a
disable isn't needed.

v2: Upstream moved and now our patch needs to remove dev_priv.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453406837-10511-1-git-send-email-paulo.r.zanoni@intel.com